1. Field of the Invention
The present invention relates to high-frequency signal lines, more particularly to a high-frequency signal line preferably for use in high-frequency signal transmission.
2. Description of the Related Art
As a conventional high-frequency signal line, a signal line described in, for example, Japanese Utility Model No. 3173143 is known. FIG. 11 is an exploded view of the high-frequency signal line 500 described in Japanese Utility Model No. 3173143.
The high-frequency signal line 500 includes a dielectric element assembly 502, a signal line 506, and ground conductors 508 and 510. The dielectric element assembly 502 is formed by laminating dielectric sheets 504a to 504c. The signal line 506 is provided on the top surface of the dielectric sheet 504b. The ground conductors 508 and 510 are provided respectively on the top surfaces of the sheets 504a and 504c. 
Furthermore, the ground conductor 508 has a plurality of openings 520 provided along the signal line 506. Accordingly, the signal line 506 overlaps alternatingly with the openings 520 and bridge portions 522. In the high-frequency signal line 500 thus configured, the characteristic impedance of the signal line 506 is lower in the areas where the signal line 506 overlaps with the openings 520 than in the areas where the signal line 506 overlaps with the bridge portions 522. Accordingly, the characteristic impedance of the signal line 506 fluctuates cyclically. As a result, in the high-frequency signal line 500, a high-frequency standing wave with a half wavelength that is approximately equal to the interval between the bridge portions 522 occurs, but there is little chance of generating a low-frequency standing wave with a half wavelength equal to about the entire length of the high-frequency signal line 500. Therefore, in the high-frequency signal line 500, the occurrence of low-frequency noise is suppressed.
Incidentally, the high-frequency signal line 500 requires the three dielectric sheets 504a to 504c. Accordingly, the high-frequency signal line 500 is demanded to be further reduced in thickness as a whole.